During the past few decades, because of cultural and social changes, women in the developed world have significantly delayed childbirth. For example, first birth rates for women 35-44 years of age in the United States have increased by more than 8-fold over the past 40 years (Ventura Vital Health Stat 47:1-27, 1989 Matthews NCHS Data Brief 2009 21:1-8). It is well known that pregnancy rates in women at 35 or more years of age are significantly lower, both naturally and with assisted reproduction. The decline in live birth rate reflects a decline in response to ovarian stimulation, reduced embryo quality and pregnancy rates, and an increased incidence of miscarriages and fetal aneuploidy. In addition, aging-associated chromosomal and meiotic spindle abnormalities in eggs are considered the major factors responsible for the increased incidence of infertility, fetal loss (miscarriage) and conceptions resulting in birth defects—most notably trisomy 21 or Down syndrome—in women at advanced reproductive ages (Henderson et al., Nature 1968 218:22-28, Hassold et al., Hum Genet 1985 70:11-17, Battaglia et al., Hum Reprod 1996 11:2217-2222, Hunt et al., Trends Genet 2008 24:86-93).
At present there is no known intervention to improve the pregnancy outcome of older female patients. In animal studies, chronic administration of pharmacologic doses of anti-oxidants during the juvenile period and throughout adult reproductive life has been reported to improve oocyte quality in aging female mice (Tarín et al., Mol Reprod Dev 2002 61:385-397). However, this approach has significant long-term negative effects on ovarian and uterine function, leading to higher fetal death and resorptions as well as decreased litter frequency and size in treated animals (Tarín et al., Theriogenology 2002 57:1539-1550). Thus, clinical translation of chronic anti-oxidant therapy for maintaining or improving oocyte quality in aging females is impractical.
Aging and age-related pathologies are frequently associated with loss of mitochondrial function, due to decreased mitochondrial numbers (biogenesis), diminished mitochondrial activity (production of ATP, which is the main source of energy for cells) and/or accumulation of mitochondrial DNA (mtDNA) mutations and deletions. As oocytes age and oocyte mitochondrial energy production decreases, many of the critical processes of oocyte maturation, required to produce a competent egg, especially nuclear spindle activity and chromosomal segregation, become impaired (Bartmann et al., J Assist Reprod Genet 2004 21:79-83, Wilding et al., Zygote 2005 13:317-23).
Heterologous transfer of cytoplasmic extracts from young donor oocytes (viz. obtained from different women) into the oocytes of older women with a history of reproductive failure, a procedure known as ooplasmic transplantation or ooplasmic transfer, demonstrated improved embryo development and delivery of live offspring. Unfortunately, however, the children born following this procedure exhibit mitochondrial heteroplasmy or the presence of mitochondria from two different sources (Cohen et al., Mol Hum Reprod 1998 4:269-80, Barritt et al., Hum Reprod 2001 16:513-6, Muggleton-Harris et al., Nature 1982 299:460-2, Harvey et al., Curr Top Dev Biol 2007 77:229-49. This is consistent with the fact that maternally-derived mitochondria present in the egg are used to “seed” the embryo with mitochondria, as paternally-derived mitochondria from the sperm are destroyed shortly after fertilization (Sutovslcy et al., Biol Reprod 2000 63:5820590). Although the procedure involves transfer of cytoplasm and not purified or isolated mitochondria from the donor eggs, the presence of donor mitochondria in the transferred cytoplasm, confirmed by the passage of “foreign” mitochondria into the offspring, is believed to be the reason why heterologous ooplasmic transfer provides a fertility benefit. Irrespective, the health impact of induced mitochondrial heteroplasmy in these children is as yet unknown; however, it has been demonstrated that a mouse model of mitochondrial heteroplasmy produces a phenotype consistent with metabolic syndrome (Acton et al., Biol Reprod 2007 77: 569-76). Arguably, the most significant issue with heterologous ooplasmic transfer is tied to the fact that mitochondria also contain genetic material that is distinct from nuclear genes contributed by the biological mother and biological father.
Accordingly, the children conceived following this procedure have three genetic parents (biological mother, biological father, egg donor), and thus represent an example of genetic manipulation of the human germline for the generation of embryos. Ooplasmic transplantation procedures that result in mitochondrial heteroplasmy are therefore now regulated and largely prohibited by the FDA. For details, see CBER 2002 Meeting Documents, Biological Response Modifiers Advisory Committee minutes from May 9, 2002, which are publicly available from the FDA and “Letter to Sponsors/Researchers—Human Cells Used in Therapy Involving the Transfer of Genetic Material By Means Other Than the Union of Gamete Nuclei”, which is also publicly available from the FDA.
Although the use of autologous mitochondria from somatic cells would avoid mitochondrial heteroplasmy, the mitochondria of somatic cells also suffer from age-related loss of mitochondrial function, due to decreased mitochondrial numbers (biogenesis), diminished mitochondrial activity (production of ATP, which is the main source of energy for cells) and/or accumulation of mitochondrial mtDNA mutations and deletions. Therefore, for women of advanced maternal age, no significant benefit would have been expected from transferring mitochondria derived from autologous somatic cells into oocytes. Moreover, a variety of stem cells are known to possess low mitochondrial activity (Ramalho-Santos et al., Hum Reprod Update. 2009 (5):553-72) and, therefore, adult stem cells were not thought to be viable sources of high activity mitochondria.